Field of the Invention
The present invention relates to a method of monitoring an output wavelength of an optical signal, and an optical signal monitoring circuit.
Background Art
In the field of optical communication, with the advance of a DWDM (Dense Wavelength Division Multiplexing) system, a wavelength spacing of individual channels becomes narrow, and highly precise control of an output wavelength of each optical signal to be handled is desired. For example, an optical transmitter described in Patent Document 1 below has a function of controlling an output wavelength output from a wavelength-variable light source element included therein. Specifically, this optical transmitter includes a first photodetector which directly receives an optical signal from the wavelength-variable light source element and converts the optical signal into an electrical signal, a second photodetector which receives the optical signal through an etalon filter and converts the optical signal into another electrical signal, and an AFC circuit which controls an output wavelength of the wavelength-variable light source element based on these electrical signals. With such a configuration, it is possible to control the output wavelength by detecting a part of the optical signal through the etalon filter using one of the two photodetectors, directly detecting a part of the optical signal using the other photodetector, acquiring wavelength information of the optical signal from a ratio of the detection results, and adjusting a set value of the wavelength-variable light source element using a value of wavelength deviation obtained from the wavelength information.
Further, a circuit configuration for monitoring an optical signal output from a semiconductor laser is shown in Patent Document 2 below. As the configuration of the circuit for monitoring an optical signal, a configuration in which a monitor current output from a light receiving element is converted into an electrical signal using a resistor and an operational amplifier is generally known, as shown in the document.    Patent Document 1: Japanese Patent Laid-Open No. 2002-185074    Patent Document 2: Japanese Patent Laid-Open No. 2002-324937
When a general configuration of the circuit for monitoring an optical signal described in Patent Document 2 above is applied to the optical transmitter described in Patent Document 1 above, the following problems occur. In other words, a positive input and a negative input of the operational amplifier included in the circuit are ideally virtually grounded, but an error is actually caused due to a process variation in the operational amplifier. This error is generated as an offset voltage in the output of the operational amplifier. Also, when the currents generated from two photodetectors are converted into voltages through the resistor, the offset voltage of the operational amplifier may be added to wavelength information to be obtained and precision of the wavelength information may be degraded.